The present invention relates to a semiconductor device mounted on an IC card and, more particularly, to a semiconductor device having a security circuit for preventing illegal access to the device.
An IC card incorporating a semiconductor device (hereinafter referred to as a chip) has recently been popularized rapidly. In a financial world, the demand for such a card is growing to use it as an electronic wallet in which user's information such as an ID number and a balance is recorded.
A semiconductor circuit is formed in the chip of the IC card. The chip includes circuit blocks of a microprocessor (MPU) and a semiconductor memory such as an EEPROM, a ROM and a RAM. The information such as the user's ID number and balance is read out of the semiconductor memory, chiefly the EEPROM through the MPU or it is written thereto. The IC card is operated as an electronic wallet by reading/writing such information from/to the chip.
The above-described chip is usually provided with a test circuit for testing the EEPROM storing the information and a test pad connected to the test circuit. Using the test pad, the circuit operation and stored information of the EEPROM are tested.
Since, however, information can be read out of/written to the EEPROM relatively easily using the test pad, it is likely that security information such as an ID number and a balance, stored in the EEPROM, will be illegally decrypted or written through the test pad. Such an illegality may cause great damage to a user or make problems.